The present invention relates to a method of producing a cylinder head provided with an exhaust port liner for an internal combustion engine, and particularly to a method of producing a cylinder head using a tubular port liner made of ceramics, and being provided with a covering layer of a refractory fiber as a heat insulating layer while the outer circumference of the layer is being cast of metal.
The present invention is intended to produce a cylinder head provided with a heat insulating port liner without damaging the port liner in the producing process and without deteriorating the heat insulating effect of a covering layer of refractory fibers.
Generally, if an internal combustion engine is improved in combustion efficiency, exhaust gas temperature is lowered, and for example, the purifying action by perfect combustion of an exhaust gas with oxidation catalyst, or the like, is degraded. Therefore it is necessary to keep the exhaust tube passage adiabatic and warm as much as possible so as not to lower the temperature of the exhaust gas.
An exhaust port liner is employed to cope with this situation, but the exhaust port portion is the place which is normally cooled by cooling water and, therefore, the deterioration in action of the purification catalyst due to temperature drop of the exhaust gas is remarkable.
In order to eliminate a disadvantage, there have been proposed a port liner in which a metal pipe made of stainless steel, or the like, is made to have a double pipe construction and another port liner in which a covering layer of refractory fibers is provided around the outer circumference thereof. The former case is insufficient in insulating effect of air between double pipes, while the latter is superior to the former case in insulating effect because it does not use metal. The latter case, however, has such disadvantages that ceramics were damaged in the producing process, or the like, and that insulating effect was deteriorated due to impregnation such as water absorption into the refractory fibers.
That is, generally, when a cylinder head is produced by hollow casting a port liner, casting is performed with core sand packed in the port liner. In this case, however, it has been impossible to avoid such accidents that the port liner made of ceramics was damaged due to thermal expansion of the core sand and that the port liner was damaged when vibrations were applied to remove the core sand out of the inside. Further, in a step after the cylinder head has been cast with metal such as, for example, an aluminum or an aluminum alloy around the port liner it is necessary to put the port liner, with the cast cylinder head into water in order to perform heat treatment such as quenching, annealing or the like, and therefore there has been such a tendency that the adiabatic effect is deteriorated because water is soaked, impregnated, and left as it is within the covering layer of refractory fibers around the ceramic port liner in the above-mentioned heat treatment step, or because inorganic and organic solutions were impregnated into the covering layer of the refractory fibers when impregnation treatment with those solutions was performed.